Cable used in the telecommunications industry, such as in telephone systems, generally requires a waterblocking material in the cable to protect the cable from water entry and/or from the longitudinal travel of water along the cable. This is true whether the cable is buried beneath the ground or laid under water. It is also sometimes required in aerial applications.
Attempts to waterproof cable such as buried cable began nearly 100 years ago and were unsuccessful in a practical sense until the introduction of plastic insulated cable during the 1950's. Specially sheathed cables, with an inner plastic jacket, aluminum and steel shield metals and an outer plastic jacket, have been used successfully. Pressurized cable also contends successfully with water problems. However, both of these approaches are deficient, the former leaves the cable vulnerable and the latter is expensive to maintain and subjects the cable to critical exposure in the event of failure of the pressurization system.
Since 1970, large quantities of cable have been filled with waterproofing compounds. This approach followed the recognition that in plastic insulated cable, the localized intrusion of water into the cable sheath is not in itself a serious problem. Disruption or deterioration of service occurs when long lengths of cable become flooded. Flooding occurs because water that penetrates into a localized opening in the cable sheath is free to channel as far as gravity allows, often hundreds of feet. Not only does this upset the capacitance balance of the transmission lines, but it introduces more potential corrosion sites in proportion to the length of wire that is wetted. Corrosion typically occurs slowly, but the useful life of water soaked wires is obviously shorter than that of dry wires.
A solution that has been widely adopted is to fill the voids in the cable with a water insoluble filling material that simply plugs the cable to channeling water. However, though the physical function of the cable filling material is straightforward, the choice of the material is not. Among the many considerations that are important for materials used in this application are the hydrophobic nature of the material, stability on aging, low temperature properties, flow characteristics at elevated temperatures, processing characteristics, handling characteristics, dielectric properties, toxicity and cost.
Materials that satisfy most of these criteria, and which have been used widely in this application, are described in U.S. Pat. Nos. 3,607,487 and 3,717,716 issued Sep. 21, 1971 and Feb. 20, 1973, respectively. These materials are essentially a petroleum jelly mixed with a polymer, usually polyethylene, to impart consistency and prevent flowing at warm temperatures.
Similar hydrophobic filling materials have been proposed for filling splice closures. For example, U.S. Pat. No. 3,879,575 issued Apr. 22, 1975 describes a mixture of a low viscosity oil, gelled by a styrene-isoprenestyrene copolymer, again with a polyethylene wax added to impart consistency and reduce slump.
More recently, an improvement over the petroleum jelly-polyethylene wax cable filling material has been disclosed in U.S. Pat. No. 4,259,540 issued Mar. 31, 1981 in the name of R. A. Sabia. This patent discloses a material which overcomes the objectionable handling characteristics of the petroleum jelly-polyethylene cable filling material. For example, because installation and maintenance of cables often requires the cable to be spliced, such splicing generally requires the isolation and removal of filling material from individual wires or optical fibers in the splice region where the cables are filled with the petroleum jelly material. Otherwise, an oily interface may form between the wire and the polyurethane material subsequently used to encapsulate the splice. This oily interface which can serve as a path for water entry into the splice can result in service-affecting trouble. Moreover, removing just sufficient material to effect the splice is time consuming and the task is generally undesirable. Further, handling at low temperatures is significantly more difficult, necessitating on occasion use of a torch to preheat the cable or the use of solvents to soften the encapsulated core. The improved material described in U.S. Pat. No. 4,259,540 overcomes the aforementioned objections to the cable filled with the petroleum jelly-polyethylene material. The improved material according to the patent is a mixture of a naphthenic or paraffinic oil having specific characteristics, a styrene-ethylene butylene-styrene (S-EB-S) triblock copolymer having a styrene-rubber ratio of from about 0.2 to 0.5 and polyethylene having a softening point of 110.degree. C. to 130.degree. C. See also U.S. Pat. No. 4,176,240 which issued on Nov. 27, 1979 in the name of R. A. Sabia.
It should be noted that the term styrene-rubber ratio, when used herein, refers to the weight ratio of the styrene block to the rubber block in the copolymer. Further, whenever the term S-EB-S is employed, it refers to a triblock copolymer whereas the term S-EB refers to a diblock copolymer.
Whereas the cable filling material of U.S. Pat. No. 4,259,540 has proved to be excellent in blocking the flow of water in a cable, it alone may not be completely suitable in meeting newly established standards for waterblocking. These standards set forth that there shall be no flow of water through an eight-foot length of cable when the length of cable is subjected to a twelve (12) foot head of water for twenty-four hours.
The patent literature also describes cables including water swellable polymers such as polyvinyl alcohol, polyacrylamides, or cellulose derivatives, which are applied to bundle wrappings or contained in moisture barriers which are spaced along the length of the cable outside of the conductor bundles and between portions of a sheath system. The area outside the core and the between portions of the sheath system is referred to as the flooding zone.
Such cables are, however, characterized by certain disadvantages and limitations. In the case of those which include one of the above-described water swellable polymers, the polymer is generally supplied in powdered or granular form. If not distributed throughout the cable core, effective water absorbence is not assured throughout that zone. The powder may be included in a tape laminate which extends longitudinally along the cable.
Using lower concentrations of the powder in the filling material compromises the water blockage capabilities of the filling material. Further, certain swelling agents such as polyvinyl alcohols and polyacrylamides do not swell quickly enough in cold water to effect proper water blockage when a cable core is only partially filled whereas filling the core completely with such agents is prohibitively expensive and causes problems with swelling in the confined space when contacted by water.
More recently, in PCT/US 90/01863 having an international publication number WO 90/12406 is disclosed a gel composition which can be used as both a filling and or an encapsulating compound. The composition is comprised of a fluid, a thickener for mixing with the fluid to form a gel matrix, and a water absorbent polymer having anionic groups attached to the polymeric backbone which is generally supplied in the form of a fine powder. This powdered hydrocarbon polymer is mixed with the dielectric gel matrix. In many cases, the dielectric gel matrix is hydrophobic and the addition of a supplementary hydrophilic substance is beneficial.
The gel composition itself provides an initial barrier to the entry of water into the confined space in which the gel is located. If water does enter the space, whether the space is the inside of a fiber optic cable, a housing or splice, or the filling or flooding zone of a telecommunications cable, the water absorbent polymer in the gel is activated and the water is absorbed. Once the water is contacted by the polymer in the gel, a highly viscous semi-solid material forms that, depending on the viscosity of the gel composition, is incapable of fluid movement.
The gel composition of the above-identified PCT document therefore plays several roles in protecting the contents or components of a confined space such as a housing or cable from water damage. First, if there is invasive moisture, the gel composition repels the water. Additionally, in the presence of water, the water absorbent polymer of the gel is activated to absorb the water, preventing its further migration.
In the PCT disclosure, it is generally preferred that the viscosity range of the gel is from about 2 centistokes at 100.degree. C. to about 90,000 centistokes at 40.degree. C. The viscosity of the composition in the PCT document must be relatively high judging from the inclusion of thickeners. Also, such relatively high viscosity should be evident from the manner of use to fill cables. The gel composition of the foregoing PCT document apparently is used to fill cables in the 20 conductor pair range. Cables today may include 3000 or more conductor pairs. Of course, a plurality of 20 pair units each could be filled and then the units assembled in a very large conductor pair size cable. However, this technique does not result in all the interstices, particularly those between the units, being filled.
What is needed and what seemingly is not available is a large conductor pair size cable which includes a waterblocking material which fills the interstices in the core and a flooding material which floods between layers of a sheath system of the cable to preserve the electrical characteristics of the cable under new waterblocking requirements. More particularly, the sought-after cable should include waterblocking compositions which not only are suitable for filling and flooding but which also may be applied in a manufacturing line on which the cable is made.